Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. 23, pages 202-203, describes a process for preparing pure perrhenic acid by firstly thermally decomposing a technical-grade ammonium perrhenate under nitrogen to form rhenium dioxide. The rhenium dioxide is subsequently converted thermally, with addition of oxygen and water, into pure perrhenic acid. Pure ammonium perrhenate is then prepared directly therefrom by means of ammonia.
The conversion of rhenium dioxide into perrhenic acid occurs via volatile dirhenium heptoxide (Re2O7) which is formed as an intermediate and on introduction into water or cocondensation together with water forms perrhenic acid. When the above-described process is employed, typical cationic impurities such as, for example, alkali metals or transition metals in technical-grade ammonium perrhenate remain as nonvolatile alkali metal perrhenates or heavy metal perrhenates in the sublimation residue such that the purification method should be very effective. However, this only applies with the proviso that no transfer of the starting material or of the residue into the receiver in which the perrhenic acid is collected can occur. The latter can be achieved only with great difficulty under practical conditions. This fact is also why the preceding decomposition of the technical-grade ammonium perrhenate into rhenium oxide is necessary. Virtually all impurities in ammonium perrhenate, both cationic and anionic in nature, can be removed by (multiple) recrystallization. An exception is potassium which cannot be removed from the ammonium perrhenate by the last-named process.
The process described in the prior art for preparing pure perrhenic acid is concerned virtually exclusively with the removal of this element. This process comprises two thermal stages and is therefore very complicated.
In “Hydrometallurgy”, vol. 89 (2007), pages 289-296, Leszczynska-Sejda et al. describe a process for preparing perrhenic acid via an ion-exchange process. Here, ammonium perrhenate solutions are converted into perrhenic acid by cation exchange. The resulting purity of the perrhenic acid is relatively good. However, the exchange process described is quite complicated and the exchange cycles must be carried out a large number of times in order to obtain sufficiently ammonium-free perrhenic acid. Furthermore, the perrhenic acid initially obtained using the best-suited ion exchanger has a very low rhenium concentration of about 17 g/l. To obtain concentrated rhenic acid having about 300 g of rhenium per liter, it is necessary to concentrate the dilute acid by evaporation of the large excess of water. To avoid considerable losses of rhenium, this must be carried out at 60° C. in a vacuum evaporator.